Numerous types of medical equipment, as is well known, are used to reconstruct an image of the interior of a subject across a plane. Examples of such medical equipment include x-ray tomographic scanners, magnetic resonance imaging scanners, and nuclear medicine emission computed tomography (ECT) scanners. Each of these scanners are used to reconstruct an image of a cross section, or slice, through a patient's body. Consecutive cross sectional images are then combined to create an image of the interior of a subject.
Tomography is a medical technique of radiographic analysis which provides an image of a particular plane of a body under examination. In one form of tomographic instrument known as a CAT scanner (computed axial tomography), an x-ray source and x-ray detectors are positioned in alignment on opposite sides of a subject under examination and simultaneously scan an edge of a cross sectional plane or slice of finite thickness extending through the subject.
Intensity of x-ray transmission through the subject is determined by sampling an electrical output of the detectors. Sampling occurs in numerous locations in the direction of a single scan. The x-ray source is then rotated a predetermined angular distance about an axis normal to the plane or slice through the subject and another scanning of the edge in a different direction is obtained. Resultant data is processed by a computer to reconstruct an image of the planer cross-section or slice through the patient's body.
In another form of image reconstruction known as magnetic resonance imaging (MRI), the body of a subject is placed within a magnetic field. When a radio-frequency at the resonance (Larmor) frequency is applied to the subject within the magnetic field, the magnetic moment of the subject's atoms which are normally in random alignment align in a north and south direction relative to the magnetic field. When the radiofrequency is terminated, the atoms return to their random alignment and in so returning will emit energy via radiofrequency at the same resonance frequency. This radiofrequency is detected via an antenna and the resultant data generated by the detection can be analyzed into contributing frequencies and processed by a computer to reconstruct an image in cross section of the patient's body. This form of image reconstruction is also known as nuclear magnetic resonance (NMR) imaging.
Image reconstructing apparatus include such CAT scanners, MRI or NMR scanners, ECT scanners, and any apparatus that receives data and processes it to create an image in cross section of the patient's body.
It is desirable at times to verify that the operation of an instrument for reconstructing an image of the interior of a subject conforms with its known capabilities. In addition, it is desirable to predetermine the capabilities of the instrument for the performance of specific examinations.
A test body (phantom) for determining the operating characteristics of a scanning tomographic analytical apparatus of the type known as a CAT scanner is disclosed in co-assigned Goodenough et al. U.S. Pat. No. 4,055,771, issued Oct. 25, 1977. The contents of this U.S. Patent in its entirety are hereby incorporated into this application in order to more fully define the state of the art to which the subject invention pertains. The test body comprises energy absorption means arranged in layered arrays extending generally parallel to a direction of projection of the x-ray beam, and means for positioning the energy absorbing means between a scanning x-ray beam and a transmission intensity detector of the tomographic apparatus.
A phantom for determining the operating characteristics of a nuclear magnetic resonance scanner is disclosed in Newman and Sierocuk U.S. Pat. No. 4,644,276, issued Feb. 17, 1987. The phantom comprises at least two test plates which include means for testing parameters of the scanner and allows the scanner to be tested in the plane in which these test plates lie.
Another phantom for determining the operating characteristics of a magnetic resonance imaging scanner is disclosed in Gray U.S. Pat. No. 4,692,704, issued Sep. 8, 1987. The phantom includes a generally tubular body containing a cylindrical stack of a plurality of leaves. Each leaf has one or more wedge shaped slices or sectors cut out for identification and reference purposes.
CAT scanners reconstruct images in fixed planes. Magnetic resonance imaging systems are not mechanically dependent in the way that CAT scanner systems are, therefore magnetic resonance imaging scanners are able to reconstruct images in any plane. A cylindrical phantom, used for testing the performance of a CAT scanner and some MRI scanners, does not provide for this capability. Therefore, the subject invention provides a spherical phantom which can measure the performance of a magnetic resonance imaging scanner (or a CAT scanner) in more than fixed planes, whereas previous phantoms have only allowed imaging in up to three test planes, X, Y and Z, mounted in a cylindrical housing. The spherical phantom of the subject invention is filled with a liquid and by inserting the test objects used in the subject invention into the liquid filled sphere, which can be rotated in all directions, variable plane positioning can be obtained.
In addition to providing multiple plane measurements, the spherical phantom of the subject invention eliminates some artifacts created by the hard corners found in cubical and cylindrical phantoms.
In addition to the imaging features provided by the subject spherical phantom, problems associated with many liquid filled phantoms involving the removal of numerous screws to open the phantoms are solved by providing a threaded joint which allows the phantom to be opened without tools. By providing a sealing means, such as an 0-ring, the seal between the two halves of the spherical phantom prevents leakage which is also a common problem with liquid filled phantoms.
Additionally, a small insert is glued or cast into the sphere to allow for longer threads to be cut through the sphere surface, preventing potential leaking problems with filling ports on the thin surface of the sphere and forming a solid base for an internal mounting post and/or a plug.
These problems associated with previous phantoms are also encountered in phantoms used to test other medical equipment used to reconstruct an image of the interior cf a subject across a plane. Thus, there continues to be a need for a test body (phantom) for determining the operating characteristics of imaging equipment. These test bodies must provide for the efficient and accurate verification of the operating characteristics of the equipment.